Hot runner manifolds are commonly used in the injection molding of plastic materials. The manifolds are used to transport the molten plastic material from an injection molding machine to various bushings or nozzle drops. The manifolds have passageways or channels in them for transport of the molten plastic material to the bushings or nozzles, and finally to the mold cavity. Heater members, or elements, are contained within or on the manifold and maintain the plastic material in the manifold at the requisite temperature for molding.
Conventional manifolds typically are made from a single block or piece of metal material. The channels or passageways in these manifolds are typically formed by drilling, such as by use of a gun drill. It is difficult to grind or smooth the interior of flow channel holes bored by gun drills, however, and the corners formed at junctions where two or more bored holes meet create additional problems. Sharply angled corners hinder the smooth flow of the plastic materials and any imperfections or defects in the smoothness of the flow channels can create distress in the material and may retain remnants of material creating difficulties in either the quality of the finished products or in cleaning the manifold after completion of the molding process.
Two-piece hot runner manifolds have been developed in an attempt to solve some of the above-identified problems. In the two-piece structures, the flow channels are formed by mating semi-circular recesses in each of the two pieces of manifold. When the two pieces are mated together, the two recesses form elongated circular flow channels.
Problems have developed with two-piece manifolds relative to securely holding the two pieces together during the molding process and in providing a smooth channel, particularly where the two surfaces of the manifold come together. Welding or bolting the two pieces of the manifold together have not always produced satisfactory results. Also, the use of a bonding or bronzing process to hold two pieces of the manifold together has not always worked satisfactorily. The bronze material, for example, can lead to protrusions of material at the parting line in the flow channels for the molten plastic. Also, voids can exist where the bronze material is not present uniformly along the parting line. Either condition can cause plastic material to be trapped in the channels producing undesirable results in the end products which are molded.
Thus, a need exists for a hot runner manifold which is an improvement over single block manifolds due to their limitations, as well as an improvement over two-piece manifolds due to their limitations. It is thus desirable and an object of the present invention to provide an improved manifold structure which eliminates or minimizes the problems mentioned above with known one and two-piece manifold structures.